Heavy-duty automobiles have various transmissions, and the transmission for a multi-gear heavy-duty automobile generally has a combined structure of a main case and an auxiliary case, and currently, the structure of main and auxiliary cases having two countershafts is a typical structure for the transmissions of the heavy-duty automobiles. In such structure, on a cross section of a partition wall of the rear auxiliary case, a rear supporting hole for a reverse countershaft of the main case is in an area of a front bearing of the countershaft of the auxiliary case, thus the rear portion of the reverse countershaft is suspended. The reverse countershaft is generally in a cantilever form being locked in the case by a nut and positioned by a boss for the reverse countershaft, and this positioning manner requires that the reverse countershaft and assembly and the transmission case are assembled to form an assembly in advance. For a transmission having a cylinder main case structure and helical gears, the countershaft is difficult to be fitted into the main case when the reverse countershaft gear is fixed.
The positioning of the front bearing of the auxiliary case countershaft is also very important, and conventionally, a reverse cup is installed at a rear of the reverse countershaft gear to axially position the auxiliary case countershaft. In such positioning manner, the reverse countershaft is in the cantilever form, it is difficult or even impossible to assemble the reverse countershaft in the transmission having the cylinder main case structure and helical gears.
With vigorous promotion of energy saving and emission reduction domestically, lightweight design of automobile is imperative, and lightweight design of transmission is a significant part of the lightweight design of automobile, and the transmission case using aluminum alloy to realize the lightweight design is also an irresistible trend. Since a space, where the reverse countershaft gear is located, forms an inverted structure inside the case, and for ensuring the feasibility of the casting, a window is generally provided at a side face of the case corresponding to the reverse countershaft gear, however, the case with windows at both side faces may significantly reduce the strength and rigidity of the transmission housing, and may cause a complex manufacturing process, increase the oil leakage fault points, and also adversely affect the appearance of the transmission.